1. Field of the Invention
The present invention relates to a wire structure, and more particularly, to a wire structure including copper or its alloy, a method of forming a wire, a thin film transistor (TFT) substrate, and a method of manufacturing a TFT substrate.
2. Description of the Related Art
A TFT substrate is used as a substrate for a liquid crystal display (LCD) having pixels in a matrix array and an organic electro luminescence (EL) display.
An LCD, which is one of the most widely used flat panel displays, includes two substrates having a plurality of electrodes and a liquid crystal layer interposed therebetween and adjusts the amount of light transmitted therethrough by applying a voltage to the electrodes to rearrange the orientations of liquid crystal molecules in the liquid crystal layer. In the LCD, TFTs are used as switching elements for controlling picture signals applied to the respective electrodes.
An organic electroluminescence (EL) display device displays a picture by electrically exciting phosphorescent organic material, and includes a driving TFT for supplying pixels with current necessary for light emission and a switching TFT.
As the display areas of LCD or the organic EL display devices increase, the gate lines and the data lines connected to the TFTs also grow longer, causing an increase in the resistance of wires. To counter signal delays resulting from the increase in resistance, gate lines and data lines can be formed of a material having as low a resistivity as possible.
Among various wiring materials, copper (Cu) is inexpensive and has relatively low resistivity. Cu has a resistivity of approximately 1.67 μΩcm, which is much lower than that of aluminum (Al), i.e., approximately 2.65 μΩcm. Therefore, in an actual process, a gate line or a data line made of Cu significantly improves the signal delay problem compared to an Al-based gate or data line.
However, since copper has poor adhesion with a lower structure such as an insulating substrate (e.g., glass substrate) or a semiconductor layer and poor resistance to chemicals, it is easily oxidized or corroded when exposed to chemicals in a subsequent process, which makes it difficult to use wire made of only copper. Thus, copper film can be used in the form of a multi-layered structure composed of copper film, an underlying barrier film, and an overlying capping film. However, when the copper film, the barrier film, and the capping film constituting the multi-layered structure are etched away in the same process using an etchant, some copper ions dissolved in the etchant may penetrate into a structure (e.g., semiconductor layer) under the multi-layered structure, thereby lowering the characteristics of the semiconductor layer, resulting in a reduction in reliability of a TFT. In addition, copper at an interface between the copper film and the capping film is selectively corroded during wet-etching for patterning or during removal of a photoresist film, thereby leading to a poor lateral profile of a wire such as with overhangs. These overhangs cause cracks in a subsequent process, thereby lowering the reliability of the wire.